1. Field of the Invention
The present invention relates to a brake system for a vehicle, comprising a liquid pressure generating means for generating a liquid pressure dependent upon a manual operation, an actuator for outputting a liquid pressure dependent upon an electrical command signal applied thereto, and an electromagnetic switchover valve means which is switchable between a first state in which a wheel brake is connected to the liquid pressure generating means upon deenergization of the electromagnetic switchover valve means, and a second state in which the wheel brake is connected to the actuator upon energization of the electromagnetic switchover valve means.
2. Description of the Prior Art
In a brake system which is conventionally known, for example, from Japanese Patent Application Laid-Open No. 65060/93, when the operation of the actuator is normal, the electromagnetic switchover valve means is energized to connect the actuator to the wheel brake. When the operation of the actuator becomes abnormal, the electromagnetic switchover valve means is deenergized to connect a master cylinder serving as the liquid pressure generating means, to the wheel brake. In the normal state, braking pressure is provided by the actuator, and the output pressure from the master cylinder is used as a fail-safe. With such known system, however, during a normal braking, the actuator is normally in operation, and the electromagnetic switchover valve means is also energized, resulting in a higher frequency of operation of each of the actuator and the electromagnetic switchover valve means. Moreover, in a condition in which braking pressure is provided by the output pressure from the actuator, a stroke accumulator for accumulating output pressure from the master cylinder is required for the purpose of providing a braking feeling.
If there is a difference in pressure between the wheel brake and the master cylinder when the electromagnetic switchover valve means is switched from the state in which the actuator is connected to the wheel brake to the state in which the master cylinder is connected to the wheel brake, a sense of incompatibility due to a pressing-up or a kick-back of a brake pedal may be produced in response to the switching of the connected state.